Biological Classification – Ultra-Detailed NEET Notes
Introduction
Biological classification is the systematic arrangement of organisms into hierarchical groups based on their similarities, differences, and evolutionary relationships. This organization facilitates the study, identification, and understanding of the vast diversity of life on Earth. The primary objectives of biological classification include:
Identification: Recognizing and naming organisms.
Characterization: Describing the features of organisms.
Classification: Grouping organisms based on similarities and differences.
Nomenclature: Assigning standardized names to organisms.
Historical Perspectives
Aristotle’s Classification
Aristotle (384–322 BC), known as the “Father of Biology”, made one of the earliest attempts at classifying organisms. He categorized animals based on the presence or absence of red blood cells:
Anaima: Animals without red blood (e.g., insects, mollusks).
Enaima: Animals with red blood (e.g., vertebrates).
He also classified plants based on their morphological characteristics into:
Herbs: Small plants with soft stems.
Shrubs: Medium-sized plants with woody stems.
Trees: Large plants with a single woody trunk.
Two-Kingdom Classification
Carolus Linnaeus (1707–1778), a Swedish botanist, proposed the Two-Kingdom Classification system in the 18th century, dividing all living organisms into:
Kingdom Plantae: Included all plants.
Kingdom Animalia: Included all animals.
Limitations:
Did not distinguish between prokaryotes and eukaryotes.
Overlooked unicellular and multicellular organisms.
Ignored differences between autotrophic (self-feeding) and heterotrophic (feeding on others) modes of nutrition.
Three-Kingdom Classification
Ernst Haeckel (1866) proposed a third kingdom, Protista, to include organisms that did not fit neatly into Plantae or Animalia, such as algae, fungi, and protozoa.
Four-Kingdom Classification
Herbert Copeland (1956) introduced the Monera kingdom to separate prokaryotic organisms (bacteria and blue-green algae) from eukaryotic Protista.
Five-Kingdom Classification
R.H. Whittaker (1969) proposed the Five-Kingdom Classification, considering factors like cell structure, mode of nutrition, reproduction, and phylogenetic relationships:
Monera
Protista
Fungi
Plantae
Animalia
Detailed Overview of the Five Kingdoms
- Kingdom Monera
Characteristics:
Cell Type: Prokaryotic (lacking a true nucleus and membrane-bound organelles).
Cell Wall: Present; composed of peptidoglycan in eubacteria.
Body Organization: Unicellular or colonial.
Mode of Nutrition:
Autotrophic:
Photosynthetic: Use sunlight to synthesize food (e.g., cyanobacteria).
Chemosynthetic: Obtain energy by oxidizing inorganic substances (e.g., nitrifying bacteria).
Heterotrophic:
Saprophytic: Feed on dead organic matter.
Parasitic: Depend on a host for nutrients, often causing harm.
Reproduction:
Asexual: Primarily by binary fission.
Genetic Recombination: Through processes like conjugation, transformation, and transduction, leading to genetic variability.
Subgroups:
Archaebacteria:
Methanogens: Produce methane; found in anaerobic environments like marshes.
Halophiles: Thrive in high salt concentrations (e.g., salt lakes).
Thermoacidophiles: Inhabit hot, acidic environments (e.g., hot springs).
Eubacteria:
Cyanobacteria: Photosynthetic, oxygen-producing bacteria (formerly known as blue-green algae).
Mycoplasma: Lack a cell wall; smallest known cells capable of independent growth.
Significance:
Ecological Roles:
Decomposers: Break down dead organic matter, recycling nutrients.
Nitrogen Fixation: Convert atmospheric nitrogen into forms usable by plants (e.g., Rhizobium).
Industrial Applications:
Fermentation: Production of yogurt, cheese, and antibiotics.
Pathogenicity: Some cause diseases in humans, animals, and plants (e.g., Mycobacterium tuberculosis causes tuberculosis). - Kingdom Protista
Characteristics:
Cell Type: Eukaryotic.
Body Organization: Unicellular, colonial, or simple multicellular.
Mode of Nutrition:
Autotrophic: Photosynthetic (e.g., algae). - Kingdom Protista
Characteristics:
Cell Type: Eukaryotic.
Body Organization: Primarily unicellular; some are colonial or form simple multicellular structures.
Mode of Nutrition:
Autotrophic: Photosynthetic protists (e.g., algae) contain chloroplasts and perform photosynthesis.
Heterotrophic:
Phagotrophic: Engulf food particles (e.g., Amoeba).
Osmotrophic: Absorb dissolved nutrients (e.g., some protozoans).
Mixotrophic: Combine autotrophic and heterotrophic modes (e.g., Euglena).
Reproduction:
Asexual: Commonly through binary fission, budding, or spore formation.
Sexual: Involves gamete formation and fusion, observed in some protists.
Major Groups:
Chrysophytes:
Includes: Diatoms and golden algae (desmids).
Characteristics:
Mostly photosynthetic.
Diatoms possess silica-based cell walls forming intricate patterns.
Major producers in aquatic ecosystems.
Dinoflagellates:
Characteristics:
Mostly marine; possess two flagella.
Cellulose plates form their cell wall.
Some species cause red tides due to rapid population growth.
Euglenoids:
Characteristics:
Mostly freshwater; possess a pellicle instead of a cell wall.
Exhibit mixotrophic nutrition (e.g., Euglena).
Slime Molds:
Characteristics:
Exist as amoeboid cells; aggregate to form a multicellular structure under adverse conditions.
Reproduce via spore formation.
Protozoans:
Characteristics:
Unicellular; primarily heterotrophic.
Subgroups:
Amoeboids: Move using pseudopodia (e.g., Amoeba).
Flagellates: Move using flagella (e.g., Trypanosoma).
Ciliates: Move using cilia (e.g., Paramecium).
Sporozoans: Non-motile; parasitic (e.g., Plasmodium).
Significance:
Ecological Roles:
Primary producers in aquatic ecosystems (e.g., phytoplankton).
Decomposers, aiding in nutrient recycling.
Economic Importance:
Some cause diseases in humans, animals, and plants (e.g., malaria by Plasmodium).
Diatoms contribute to petroleum formation. - Kingdom Fungi
Characteristics:
Cell Type: Eukaryotic.
Body Organization: Unicellular (e.g., yeasts) or multicellular (e.g., molds, mushrooms).
Cell Wall: Composed of chitin.
Mode of Nutrition: Heterotrophic by absorption.
Saprophytic: Feed on dead organic matter.
Parasitic: Obtain nutrients from living hosts, often causing disease.
Mutualistic: Engage in symbiotic relationships (e.g., mycorrhizae with plant roots).
Reproduction:
Asexual: Through spore formation (e.g., conidia, sporangiospores), budding, or fragmentation.
Sexual: Involves plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei), and meiosis, leading to spore formation.
Major Groups:
Phycomycetes:
Includes: Molds like Rhizopus (bread mold) and Mucor.
Characteristics:
Aseptate (coenocytic) hyphae.
Asexual reproduction by sporangiospores; sexual reproduction by zygospore formation.
Ascomycetes (Sac Fungi):
Includes: Yeasts (Saccharomyces), morels, and truffles.
Characteristics:
Septate hyphae.
Asexual reproduction by conidia; sexual reproduction by ascospores within asci.
Basidiomycetes (Club Fungi):
Includes: Mushrooms, puffballs, and bracket fungi.
Characteristics:
Septate hyphae.
Sexual reproduction by basidiospores on basidia; asexual spores generally absent.
Deuteromycetes (Fungi Imperfecti) – The Imperfect Fungi
Characteristics:
Known as “Imperfect Fungi” because their sexual reproduction (perfect stage) is not yet observed.
Only asexual reproduction by conidia is known.
Once their sexual stage is discovered, they are reclassified into Ascomycetes or Basidiomycetes.
Mycelium is septate and branched.
Most are saprophytes or parasites; some are decomposers of leaf litter and organic debris.
They show asexual reproduction by conidia, chlamydospores, or oidia.
Examples:
Penicillium (important for antibiotic production – Penicillin).
Aspergillus (produces aflatoxins, used in fermentation).
Alternaria (causes leaf spot disease in plants).
Colletotrichum (causes anthracnose disease in crops).
Trichoderma (biocontrol agent in agriculture).
Significance of Deuteromycetes:
Beneficial Roles:
Biocontrol Agents: Trichoderma species help suppress plant pathogens.
Industrial Uses: Aspergillus species produce citric acid, gluconic acid, and enzymes like amylase and protease.
Antibiotics: Penicillium produces the life-saving antibiotic Penicillin.
Harmful Effects:
Many plant pathogens cause diseases in crops, reducing yield.
Aspergillus flavus produces aflatoxins, which are carcinogenic and contaminate food.
Some species cause skin infections in humans.
- Kingdom Fungi
Characteristics:
Cell Type: Eukaryotic.
Body Organization: Unicellular (e.g., yeasts) or multicellular (e.g., molds, mushrooms).
Cell Wall: Composed primarily of chitin.
Mode of Nutrition: Heterotrophic by absorption.
Saprophytic: Feed on dead organic matter.
Parasitic: Obtain nutrients from living hosts, often causing diseases.
Mutualistic: Engage in symbiotic relationships (e.g., mycorrhizae with plant roots).
Reproduction:
Asexual: Through spore formation (e.g., conidia, sporangiospores), budding, or fragmentation.
Sexual: Involves plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei), and meiosis, leading to spore formation.
Major Groups:
Phycomycetes:
Includes: Molds like Rhizopus (bread mold) and Mucor.
Characteristics:
Aseptate (coenocytic) hyphae.
Asexual reproduction by sporangiospores; sexual reproduction by zygospore formation.
Ascomycetes (Sac Fungi):
Includes: Yeasts (Saccharomyces), morels, and truffles.
Characteristics:
Septate hyphae.
Asexual reproduction by conidia; sexual reproduction by ascospores within asci.
Basidiomycetes (Club Fungi):
Includes: Mushrooms, puffballs, and bracket fungi.
Characteristics:
Septate hyphae.
Sexual reproduction by basidiospores on basidia; asexual spores generally absent.
Deuteromycetes (Imperfect Fungi):
Includes: Fungi where sexual reproduction has not been observed (e.g., Penicillium, Aspergillus).
Characteristics:
Reproduce asexually by conidia.
Once sexual reproduction is observed, they are reclassified into appropriate groups.
Significance:
Ecological Roles:
Decomposers: Break down complex organic matter, recycling nutrients in ecosystems.
Symbiotic Relationships: Form mutualistic associations like lichens (with algae) and mycorrhizae (with plant roots), enhancing nutrient absorption.
Economic Importance:
Industrial Applications: Used in fermentation processes (e.g., brewing, baking) and production of antibiotics (e.g., penicillin from Penicillium).
Food Industry: Edible fungi like mushrooms are consumed worldwide.
Pathogenicity: Some fungi cause diseases in plants (e.g., rusts, smuts) and humans (e.g., athlete’s foot, ringworm). - Kingdom Plantae
Characteristics:
Cell Type: Eukaryotic.
Body Organization: Multicellular with specialized tissues and organs.
Cell Wall: Composed of cellulose.
Mode of Nutrition: Primarily autotrophic; perform photosynthesis using chlorophyll pigments.
Major Groups:
Algae:
Characteristics:
Simple, autotrophic organisms; mostly aquatic.
Can be unicellular (e.g., Chlamydomonas) or multicellular (e.g., Ulva).
Significance:
Primary producers in aquatic ecosystems.
Some are used as food supplements (e.g., Spirulina).
Bryophytes:
Includes: Mosses and liverworts.
Characteristics:
Non-vascular; lack true roots, stems, and leaves.
Require moist environments for reproduction.
Significance:
Play a role in soil formation and preventing erosion.
Pteridophytes:
Includes: Ferns, horsetails.
Characteristics:
Vascular plants with true roots, stems, and leaves.
Reproduce via spores; do not produce seeds.
Significance:
Contribute to the formation of coal deposits over geological time.
Gymnosperms:
Includes: Conifers like pines and firs.
Characteristics:
Seed-producing plants; seeds are exposed (not enclosed in fruits).
Typically have needle-like leaves and deep root systems.
Significance:
Provide timber, resins, and other economically important products.
Angiosperms (Flowering Plants)
Key Characteristics:
Most diverse and advanced group of plants.
Seeds enclosed within fruits (contrast to gymnosperms, where seeds are naked).
Vascular system well-developed: Xylem has vessels, and phloem has sieve tube elements.
Double fertilization: Unique feature where one sperm fertilizes the egg to form the zygote (2n), and the other sperm fuses with the two polar nuclei to form the triploid (3n) endosperm.
Flowers as reproductive structures: Highly specialized reproductive organs to attract pollinators.
Classification of Angiosperms
Monocots (Monocotyledons):
Examples: Rice, wheat, maize, banana, palm.
Characteristics:
Single cotyledon (seed leaf).
Parallel venation in leaves.
Fibrous root system.
Floral parts usually in multiples of three.
Dicots (Dicotyledons):
Examples: Mango, sunflower, pea, rose.
Characteristics:
Two cotyledons.
Reticulate (net-like) venation in leaves.
Taproot system.
Floral parts usually in multiples of four or five.
Significance of Angiosperms
Ecological Role: Provide oxygen through photosynthesis, food for herbivores, and habitat for many organisms.
Economic Importance:
Major food sources (rice, wheat, fruits, vegetables).
Timber production (Tectona grandis – teak).
Medicinal plants (e.g., Rauwolfia, Catharanthus).
Ornamental plants (Rosa, Tulip).
Significance of Kingdom Plantae:
Ecological Roles:
Primary producers in most ecosystems; convert solar energy into chemical energy through photosynthesis.
Produce oxygen essential for the survival of aerobic organisms.
Provide habitat and food for a multitude of organisms.
Economic Importance:
Source of food, timber, fiber, medicines, and ornamental plants.
Play a crucial role in agriculture and horticulture.
- Kingdom Animalia
Characteristics:
Cell Type: Eukaryotic.
Body Organization: Multicellular with specialized tissues and organs.
Cell Wall: Absent.
Mode of Nutrition: Heterotrophic; rely on other organisms for nourishment.
Reproduction: Primarily sexual; some exhibit asexual reproduction.
Movement: Most are motile at some stage of their life cycle.
Major Phyla:
Porifera:
Includes: Sponges.
Characteristics:
Simplest multicellular animals.
Lack true tissues and organs.
Possess pores (ostia) through which water flows.
Significance:
Play a role in aquatic ecosystems as filter feeders.
Cnidaria:
Includes: Jellyfish, corals, and sea anemones.
Characteristics:
Radial symmetry.
Possess specialized cells called cnidocytes for capturing prey.
Significance:
Form coral reefs, which are vital marine ecosystems.
Platyhelminthes:
Includes: Flatworms like planarians and tapeworms.
Characteristics:
Bilateral symmetry.
Acoelomate (lack a body cavity).
Significance:
Some are parasitic and can cause diseases in humans and animals.
Nematoda:
Includes: Roundworms.
Characteristics:
Bilateral symmetry.
Pseudocoelomate (have a false body cavity).
Significance:
Many are parasitic, affecting plants, animals, and humans.
Annelida:
Includes: Earthworms and leeches.- Triploblastic and coelomate (true body cavity present).
- Body segmentation: Metameric segmentation (externally and internally segmented body).
- Locomotion: Presence of setae, parapodia, or suckers in different species.
- Circulatory System: Closed circulatory system with hemoglobin in blood.
- Excretion: Nephridia for osmoregulation and excretion.
- Nervous System: Ventral nerve cord with paired ganglia.
- Reproduction:
- Sexual reproduction.
- Earthworms are hermaphrodites but show cross-fertilization.
- Some annelids (e.g., Nereis) are dioecious (separate sexes).
- Significance:
- Earthworms help in soil aeration and fertility (hence called ‘farmer’s friend’).
- Leeches are used in medicinal therapy (hirudotherapy).
- Kingdom Animalia (Continuation)
Arthropoda:
Includes: Insects, arachnids (spiders, scorpions), crustaceans (crabs, lobsters), and myriapods (centipedes, millipedes).
Characteristics:
Largest phylum in the animal kingdom.
Bilateral symmetry with segmented bodies.
Exoskeleton made of chitin.
Jointed appendages for movement, feeding, and sensory functions.
Open circulatory system.
Significance:
Important pollinators (bees), decomposers, and a food source for many animals.
Some are agricultural pests or vectors of diseases (e.g., mosquitoes transmit malaria, ticks spread Lyme disease).
Mollusca:
Includes: Snails, octopuses, squids, clams.
Characteristics:
Soft-bodied animals, often with a calcium carbonate shell.
Bilateral symmetry with a muscular foot, mantle, and visceral mass.
Open circulatory system (except cephalopods like octopuses and squids, which have a closed circulatory system).
Radula (a rasping organ) in most, except bivalves.
Significance:
Economically important as food sources (e.g., oysters, mussels).
Some produce pearls (e.g., Pinctada).
Echinodermata:
Includes: Starfish, sea urchins, sea cucumbers.
Characteristics:
Radial symmetry in adults, bilateral symmetry in larvae.
Water vascular system used for locomotion, feeding, and respiration.
Calcareous endoskeleton.
Regenerative abilities in some species (e.g., starfish can regrow arms).
Significance:
Play a role in marine ecosystems by controlling algal populations.
Some are harvested for food (e.g., sea cucumbers).
Chordata:
Includes: All vertebrates and some invertebrates like tunicates and lancelets.
Characteristics:
Possess a notochord at some stage of life.
Dorsal hollow nerve cord.
Pharyngeal gill slits.
Post-anal tail.
Divisions of Chordata:
Urochordata (Tunicates): Notochord present only in the larval stage.
Cephalochordata (Lancelets): Notochord persists throughout life.
Vertebrata: Notochord is replaced by a vertebral column in adults.
Classes of Vertebrata:
Cyclostomata: Jawless vertebrates (e.g., lampreys, hagfish).
Chondrichthyes: Cartilaginous fishes (e.g., sharks, rays).
Osteichthyes: Bony fishes (e.g., salmon, tuna).
Amphibia: Can live both in water and on land (e.g., frogs, salamanders).
Reptilia: Cold-blooded, lay shelled eggs (e.g., snakes, crocodiles).
Aves: Birds with feathers, beaks, and hollow bones (e.g., eagles, sparrows).
Mammalia: Have mammary glands, hair, and three middle ear bones (e.g., humans, lions).